A crime-preventing infrared sensor of the passive type introduces infrared rays incident on a monitoring window serving as the cover thereof into an infrared detecting means such as a pyroelectric element through an optical system such as a reflecting mirror, and detects an intruder by amplifying and distinguishing a signal outputted from the pyroelectric element, thereby giving an alarm. In principle, the sensor detects an intruder by detecting a sudden change of infrared rays incident on the pyroelectric element, thus generating an output. Therefore, if the optical system through which infrared rays pass and is incident on the pyroelectric element fails, for example, if the monitoring window is shielded or if the reflecting mirror is dislocated, the infrared rays radiated from the intruder cannot be detected, that is, the sensor does not function. In addition, when the infrared detecting means or the signal processing circuits provided with the sensor fails, the sensor does not function either. In order to detect an abnormality which occurs in the infrared detecting means or the signal processing circuits, Japanese Utility Model Laying-Open Publication No. 60-104993 discloses a method for self-diagnosing such an abnormality. According to this method, an infrared emitting source is provided in a sensor so as to perform a self-diagnosis by irradiating infrared rays to an infrared detecting means provided in the sensor. However, a mechanism for effectively monitoring the abnormality of the optical system such as an obstruction thereof in particular has not been provided.
The reason for requiring a mechanism of effectively monitoring the abnormality of the optical system provided with the crime-preventing infrared sensor is as follows: This type of a crime-preventing system operates ordinarily at night and holidays when there is none in charge in an institution or the like, whereas the signal processing circuits of the sensor, preferably, does not receive a signal from the infrared detecting means when there are persons in charge in the institution or the like. Thus, a preliminary intruder who approaches the sensor can obstruct the visual field of the sensor, for example, he can optically shield the window of the sensor. In order to solve this problem, the inventors have recently developed a system for monitoring the abnormality of the optical system of the sensor of this type. According to the system, the visual field of an infrared detecting element is shifted from the ordinarily monitoring field to a different visual field as the reference visual field or the window of the sensor is forcibly shielded with a surface-masking element, whereby the probable change of the output from the infrared detecting element is measured. If there is no substantial change in the output, the sensor decides that an abnormality has occurred and gives an alarm. That is, the sensor decides that the visual field of the window is already in a condition similar to the reference visual field or masked so that infrared rays are prevented from passing the window, or the visual field has been substantially obstructed. However, this method cannot be performed unless a masked state of the window is different in a certain extent from the visual field to be monitored in a normal period when the infrared rays are being monitored by the sensor.